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NASA Technical Memorandum 82932

(NASA-TM-8233,2) A REMOTE AUGMENTCR LIFT N83-12087SYSTEM VITH A TURBINE 5UASS ENGINE (NASA)12 p HC A02/MF A 1̂,1 csci 21E

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A Remote Augmentor LiftSystem With a TurbineBypass Engine-

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Laurence H. Fishbach and Leo C. Franciscus V%14 No V 19 ^?^,Lewis Research Center CY)

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Cleveland, Ohio

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ESS DEPT

Prepared for the

Thirtoc!nth Congress of the International Council of Aeronautical Sciencesand Alycraft Systems and Technology Meetingsponsored by the American Institute of Aeronautics and AstronauticsSeattle, Washington, August 22-27, 1982 OD

NASA

A REMOTE AUGMENTS LIFT SYSTEM WITH A TURBINE BYPASS ENGINE

Laurence H. Fishbach and Leo C. Franciscus

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

ABSTRACT I. INTtODUCTION

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A study of supersonic vertical takeoff orlanding (VTOL) fiahter aircraft employing twoengine types, a conventional medium bypassratio turbofan, and a turbine bypass turbojetwas carried out. The aircraft assumed was aclipped delta wing with canard configuration.A VTOL deck launched intercept, DLI, missionwith Mach 1.6 dash and cruise segments wasused as the design mission. Several alternatemissions requiring extended subsonic capabili-ties were analyzed. Comparisons were madebetween the turbofan (TF) and the turbinebypass turbojet (TBE) engines in airplanetypes using a Remote Augmented Lift System,RALS and a Lift plus Lift Cruise system(L+LC). The figure of merit was takeoff grossweight for the VTOL DLI mission.

The results of the study show that theturbine bypass turbojet and the conventionalturbofan are competitive engines for both typeof aircraft in terms of takeoff gross weightand range. However, the turbine bypass turbo-jet would be a simpler engine and may resultin more attractive life cycle cos"s andreduced maintenance. The RA I .$ and L+LC air-

plane types with either TBE or TF engines haveapproximately the same aircraft takeoff gross

weight.

NOMENCLATURE

Afterburnerbypass ratiodeck launched interceptfan pressure ratiofeetgravitational gas constantpoundlift plus lift-cruiseMach numbermaximumminimumnautical mileoverall pressure ratiooptimumdegrees Rankinesecondspecific fuel consumptionsea levelturbine bypass enginetemperatureturbofantakeoff gross weightcorrected airflow rate

ripts

massforce

Providing a vertical takeoff or landing(VTOL) capability to high speed aircraft pos-ses many challanges in aircraft/propulsionsystem integration. This is especially truefor supersonic aircraft. This type of air-craft requires a low frontal area whereas liftsystems tend to increase the frontal area.,Some of the concepts that have been studiedthus far l , 2 are shown in figure 1. TheVATOL concept requires the aircraft to takeoffand land in a vertical attitude and may beobjectionable from the pilot's viewpoint. Thelift plus lift-cruise (L+LC) system carriesdedicated lift engines. This would tend toimprove engine performance at cruise. Sincethey would not be oversized for takeoff, theyneed not be throttled back drastically atcruise with resulting penalties in specificfuel consumption. However, unless the liftengines can be used for other flight condi-tions they represent a weight penalty. Also,the two engine types may result in higher life

cycle costs.

The remote augmentor lift system (RALS) infigure 1 is powered by a turbofan engine. Theengine duct flow is directed to the remoteburners during vertical takeoff and landing.This system is less compact than the L+LCsystem because of the RALS ducting. Also, itN,ould have a hot footprint during VTOL opera-

tion,

The tandem fan system is similar to remotelift fan systems. Separate air intakes areprovided for the front fan and for the mainengine when in VTOL operation. An attractivefeature of this system is that the front fanis used during both VTOL and cruise flight.

The turbojet would be a compact engine fora VTOL aircraft. However, an undesirableproblem in using the turbojet is that part ofthe hot exhaust gas must be ducted to the

forward lift point. A mean: of avoiding thisproblem is provided by the turbine bypassengine (TBE). This concept was first reported

by Boeing in their supersonic cruise airplanestudies contracted by NASA-Langley. In thisconceptual turbojet some of the compressordischarge air is bypassed around the burnerand turbine and reinjected into the nozzle.For aircraft requiring wide variations inengine power, the turbine bypass provides abetter cycle match and improved performance.

Studies have been made of the turbine bypassturbojet(TBE) for a commercial supersonictransport.

Since the compressor discharge air of theTBE provides an attractive air supply for the

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RALS during VTOL, in-house studies of thisconcept have been carried out at NASA-Lewis.In these studies, the TBE and a convzntionalmixed flow turbofan were analized it both theRALS and L+LC airplane systems. Engine per-formance and missions studies were performedfor th.cse engine concepts. The ;potential ofthe engines was assessed in terms of the per-formance of an advanced supersonic VTOLfighter. This paper provides the results ofthese studies.

inlet and nozzle drags. The inlet dragsinclude cowl pressure drag, bypass drag, andspillage drag. Nozzle performance includesthe boattail drag.

The installed propulsion system weightincludes the engine, inlet, and nozzle. Thepropulsion system weight was calculated usingan engine weight computer code.5

III. DISCUSSION

II. METHOD OF ANALYSIS

The airplane used for the study is shownin figure 2. The baseline aircraft has twomain engines. For the RALS, main engine airis ducted forward to a remote augmentor forvertical thrust in addition to the vectoredthrust of the two main engines. For the L+LCsystem, one scaled XL99 lift engine is locatedforward for the front thrust. The location ofthe front thrustor was adjusted for center ofgravity. Reaction controls powered by mainengine air are located at the wing tips.These provide pitch, yaw and roll control bymodulating thrust vectors. As shown in figure2 the airplane is a clipped delta wing withcanard configuration. The weight and dimen-sions of the airplane vary with propulsionsystem type and mission constraints.

The five missions included in the studyare shown in figure 3 and 4. As indicated inthe figures, the deck launched intercept andcombat air patrol missions are for VTOL. Theother three missions are for short takeoff orlanding (STOL). The airplane was designed forthe deck launched intercept (DLI) mission(figure 3). The design was held fixed for theremaining missions and the takeoff weightadjusted for the fuel and weapons required foreach mission. A fuel reserve of 5 percent wasassumed in the study.

The airplane/mission calculations wereperformed with the NASA Lewis Airplane MissionAnalysis Code (AMAC) which computes the vol-umes, dimensions, weight, and aerodynamics ofthe airplane and "flies"- it over the pres-cribed mission. The airplane and engine weresized to meet the design constraints listed infigure 3. The first three constraints aresatisfied by engine sizing. The specificexcess oower (PS) goal and the one minuteacceleration from Mach 0.8 to Mach 1.6 at35000 feet are usually the most critical con-straints and engines sized for these two metthe other constraints including VTO. The lastconstraint (6.2 g's at Mach 0.6, 10:10 -feet)was satisfied by adjusting the wing loading.

The uninstalled engine performance wasfirst calculated without inlet and nozzledrags using the Navy-NASA Engine Program

(NNEP). 4The engine component aerodynamiccharacteristics, efficiencies, and coolingrequirements used in the program are compat-ible with a mid-1990's technology level. Theinstalled engine performance is the unin-stalled performance adjusted for the

The Turbine Bypass Engine

For most aircraft turbine engines theturbine is choked for nearly all operatingconditions. Therefore, for a fixed turbine,the turbine corrected airflow will be constantfor nearly all operating conditions. In aconventional turbojet, the compressor willoperate at pressure ratios and airflows tomatch the constant value of turbine correctedairflow. This places limitations on thethrottle excursions the turbojet can achieve.At high throttle (high turbine inlet tempera-ture) the compressor operating point movestoward the surge region. At low throttle thecompressor operates at low pressure ratioswhich deteriorate engine performance. Onemeans of reducing these restrictions is avariable area turbine. This permits the tur-bine corrected airflow to vary, permittingwider excursions in throttle without affectingthe compressor noerating point. The objectiveof the turbine bypass concept is very similarto that of the variable area turbine. How-ever, instead of varying the turbine area, theturbine airflow is varied. Figure 5 shows aschematic of this concept for a single-spoolturbojet. The compressor is matched to anundersized turbine and provision is made forbypassing some compressor discharge air aroundthe burner and turbine and into the nozzle.As shown in the figure, the turbine inlettemperature for zero bypass is 21000R. Asthe turbine inlet temperature is increased,the bypass airflow is increased. The actualturbine airflow is reduced to maintain a con-stant turbine corrected airflow. In thisexample, the compressor operates at a singlepoint for turbine inlet temperature variationsfrom 2100 OR to 32600R. In addition to theengine performance benefits provided by theTBE, this concept js an attractive alternativefor the remote augmentor lift system for VTOLaircraft.

Propulsion Systems

As mentioned before, the TBE and a conven-tional, mixed flow turbofan were studied forboth the RALS and L+LC airplane types. Theengine cycle characteristics for these enginesare provided in Table 1. Schematics of thepropulsion system arrangements for the RALSsystems are shown in figure 6. For the RALS/turbofan system the bypass air is supplied tothe remote burner where the air is heated to3260OR during VTOL operation. For thissystem the RALS supplied 30 to 50 percent

of the total lift. For other flight condi-tions the engines operate as mixed flow turbo-fans, For the RALS/TBE system the compressorbypass air is directed to the remote burnerfor VTOL and to the engine nozzle at otherflight conditions. During vertical takeoffthe engines are operated at maximum power andthe amount of bypass air going to the remoteburner is a maximum. This amounts to about 20percent of the engine airflow in this exam-ple. The RALS provides about 17 percent ofthe total lift in the RALS/TBE system, Forother flight conditions where high power isrequired such as acceleration and combat thebypass air is injected into the engine noz-zle, As indicated in figure 6, the duct sizesfor the RALS/TBE would be about 1/3 the sizeof those for the RALS/turbofan system.

For the L+LC propulsion systems, the TBEor turbofan engines are the main engines andthe performance and weight characteristics ofthe XI.99 are used for the lift engine. Thelift engine is sized to provide 30 percent ofthe total lift for VTOL operation.

Figure 7 shows a comparison of the turbinebypass engine (TBE) and turbofan engine (TF)performance at Mach 1.6 and 0.8. Some typicaloperating points for a DLI mission are alsoshown in the figure. The indicated climbthrust is the climb throttle setting at Mach1.6. About 65' of the usable fuel is consumedduring the three flight segments at Mach 1.6.For climb and combat the TBE has about 13%lower fuel consumption than the turbofan. Fordash and the Mach 0.8 cruise the fuel consump-tion of the two engines is about the same.The better supersonic SFC's of the TBE lead tolower overall fuel consumption compared to theturbofan. As shown in Table 1, the drythrust/engine weight (FN/W) ratios of the TBEis better than that of the TF and the after-burning FN/ 14 ratios are nearly the same. It

will be shown later that the heavier engineweight of the TBE compared to the turbofan(for the same airflow) offsets this advantageto some extent.

Mission Results

Figure 8 shows a comparison of the TBE andturbofan engines in L+LC aircraft in terms oftakeoff gross weight (TOGW). Both dry andafterburning engines are shown. As indicatedin the figure, the aircraft are sized for theVTOL DLI mission and the comparisons in thisfigure are for this mission. The climb thrustof the dry turbofan is marginal for this mis-sion resulting in large engines and excessivefuel consumption, The aircraft with dry tur-bofans is about 85% heavier tnan the dry TBEaircraft. Afterburning does not improve theTBE aircraft significantly (about 8A reducedTOGW), but results in large improvements tothe turbofan aircraft. This shows that theturbofan requires afterburners, but the TBEmay not need afterburners resulting in a muchsimpler propulsion system. However, when bothengines are compared with afterburners theTON of the turbofan aircraft is only slightly

larger than the TBE aircraft.

Figure 9 shows a comparison of aircraftTOGW for RALS and L+LC systems with TBE andturbofan engines. All of these enginesexcept

the XL99 ` ' f t engine are equipped with after-burners. The high thrust/weight ratio oP' theXL99 lift engine (about 14 installed) providesa lightweight lift system competitive with theRALS. As seen in the figure the RALS and theL+LC propulsion system result in about thesame TOGW. It should be emphasized that theturbofan requi res afterburners for both RALS

and the L+LC aircraft to perform the DLI and

CA • missions ;,rile the TBE does not. This is*due to the climb to supersonic speed segmentof this mission.

Figure 10 compares the propulsion systemsfor the alternate missions. Since the largeTOGW of the airplanes with dry turbofans(figure 8) indicate dry turbofans are notsuitable for this type of airplane, onlyafterburning turbofans are considered in thiscomparison.

In comparing the dry TBE with the after-burning TBE for both RALS and L+LC aircraft,it is seen that the dry TBE is better than theafterburning TBE for Combat Air Patrol VTOL(CAP) and Strike. Engines sized for thesupersonic Deck Launched Intercept missiongive adequate power without afterburning forthe subsonic CAP and Strike missions, Theafterburning engines are smaller and lighterand operate at better SFC's during dry opera-tion (not throttled back as far) than the dryTBE's. However, the use of afterburningduring climb and combat results in excessivefuel consumption and less range. For SubsonicSurveillance about 50% of the mission fuel isused during loiter and for the Ferry missionabout 85% of the mission fuel is used forsubsonic cruise. Both the dry and after-burning TBE's operate dry for these missionsand the loiter time and range about the same.

Ln comparing the turbofan with the TBE, itis seen that the turbofan does somewhat betterthat the TBE on all of the alternate mis-sions. Since these mission are all subsonic,the SFC's of the turbofan are better thanthose of the TBE (figure 7) resulting in bet-ter range and loiter capabilities.

In comparing the RALS and L+LC aircraftfor the alternate missions both systems pro-vide about the same alternate mission capabil-ities.

CONCLUDING REMARKS

The turbine bypass engine and a mediumbypass ratio mixed flow afterburning turbofanare competitive engines for a VTOL aircraft in

terms of takeoff gross weight. Afterburningprovides a small benefit for the TBE, but isrequired in the turbofan to make it competi-tive with the TBE. For the RALS system theTBE would result in smaller duct sizes andlead to less complexity. Since the TBE does

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not need an afterburner for either the RALS orthe L+LC aircraft and being a simpler enginethan the turbofan it may be a more attractiveengine in terms of life cycle costs. Compari-sons of the RALS and L+LC systems show thatboth provide about the same takeoff thrust/weight ratios and result in about the sameaircraft takeoff gross weight.

REFERENCES

1. W. R. Boruff and A. J. Roch, "Impact ofMission Requirements on V/STOL PropulsionConcept Selection," Journal of Aircraft,Vol, 18, No. 1, pp. 43-50, January 1981.

2. R. W. Luidens, G. E. Turney and J. Allen,"Comparison of Two Parallel/Series FlowTurbofan Propulsion Concepts for Super-sonic V/STOL," AIAA Paper 81-2637,December 1981.

3. L. C. Franciscus, "Turbine Bypass Engine -A New Supersonic Cruise Propulsion Con-cept," National Aeronautics and SpaceAdministration TM-82608 (1981).

4. L. H. FishGach and M. J. Caddy, "NNEP -The Navy-NASA Engine Program," NationalAeronautics and Space Administration TMX-71857 (1975).

5. E. Onat and G. W. Klees, "A Method toEstimate Weight and Dimensions of Largeand Small Gas Turbine Engines FinalReport," National Aeronautics and SpaceAdministration CR-159481 (1979).

TABLE I - ENGINE CHARACTERISTICS

TBE TF

ENGINE CYCLE DESCRIPTION

W s/:&/ 6, lbm/sec 175 175

FPR --' 3

OPR 15 15BPR --- 1.0

MAX CET, °R 3260 3260

MAX AB, °R 3260 3260

MAX RALS TEMP,, °R 3260 3260

ENGINE WEIGHT

Engine + Nozzle + RALS, lbm 2755 2329

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